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Common Terms used in the Rubber Industry

MOULD, TOOL, DIE: All of these words can be used to describe the steel or aluminium block that forms the shapes that the rubber is forced into before curing. Moulds can vary in complexity from two plate compression moulds to large multi-cavity moulds with several cores, plates and extensive hydraulics and ejector systems. The more automated the mould the more expensive it is, however, the parts moulded are generally cheaper.
BASE POLYMER: A rubber compound is a mixture of ingredients designed to meet certain performance specifications. A compound is usually, but not always, based on a single polymer. See the polymer selection section for the main characteristics of each type of polymer.
CURES: Each time the mould fills with rubber compound and cooks is a cure or shot.
CAVITIES: The number of components that can be moulded in one cure.
CYCLE TIME: May also be called the cure rate - the length of time that it takes for one cure to be completely moulded.
UNLOADING: The removal of the parts from the mould. This can be automated with a variety of robotics, hydraulic wipers or moving cores or may be completed manually using tools and compressed air.
LOADING: In a compression die the rubber is weighed to the correct shot weight and cut into pieces and fitted into the cavities in the die. If inserts are used this is usually done manually as well. In injection machines the rubber is injected into the cavities through a runner system.
RUMBLING: The NRM term for the cryogenic deflashing process. Flash is removed from parts by being tumbled in a liquid nitrogen environment. The excess flash is frozen and is knocked off in the tumbling process.
FLASH: Rubber that overflows from the mould cavity while curing. This is generally removed by cryogenic deflashing, grinding or trimming manually depending on the application and the part.
BATCH: When a compound is mixed to a recipe this results in a batch of compound. The batch size is determined by the mixing method - there is an optimal size for a batch of rubber determined by the mixing method.
MILLING OR MIXING: The mixing of the ingredients in a compound formulation by either an Internal Mixer, where the compound is mixed by two meshing rotors in an enclosed case, or by open mill mixing, adding the ingredients carefully between two steel rollers.
After mixing the batch of uncured rubber compound must then pass Quality Assurance tests before it is available for using in one of the moulding methods.
Compound Testing
RHEOMETER TEST: A routine test performed on every batch of compound before it is released to the manufacturing department. A read out on a graph confirms that the mix meets standardised curing properties for that compound.
SPECIFIC GRAVITY: The specific gravity of the mixed compound can be calculated from the specific gravity of the components. Each compound will have a reasonably unique SG and this measure can be used to indicate if any of the ingredients have been left out of the mix. This test is performed if the rheometer test indicates a problem with the batch of rubber.
HARDNESS: A routine test performed on every batch before it is released to production. The hardness of a rubber compound is measured in the Shore A hardness range. Compounds at both ends of the hardness scale can be difficult to mix and mould. A reading of 20 is quite soft and a rubber compound of 80+ is very hard.
COMPRESSION SET: Determined for the original certification of the compound. The ability of the cured compound to return to its original size and shape after deformation.
TENSILE STRENGTH: Used to determine the strength of the rubber compound. A dumbbell shaped section of the compound is stretched to failure point.
OZONE RESISTANCE: Rubber strips under elongation pressure are exposed to ozone for a set period of time.
AGING: Tensile strength tests are performed after accelerated heat or light aging to determine the ability of the compound to resist aging.
ELECTRICAL CONDUCTIVITY: Rubber compounds can be designed to be conductive, non-conductive, insulation resistance or anti-static. Tests are used to verify performance.
RESISTANCE TO LIQUIDS: Many rubber articles come into contact with liquids during their service life. The compounds from which these items are made must be tested to ensure that they perform satisfactorily. Resistance to oils, petrol, water are commonly tested.
ABRASION RESISTANCE: This test measures the ability of a rubber to resist the wearing away of its surface.
Components of a Compound Recipe
CURATIVES: Chemicals which achieve cross-linking of the long chain rubber polymer. Sulphur was the first to be discovered and is still commonly used.
ACCELERATORS: Chemicals which vary the speed and timing of the curing reaction.
FILLERS: Relatively inert chemicals, such as clays, which increase the bulk of the compound and reduce the cost.
PLASTICISERS: Can be added to some compounds to aid processability, particularly for injection mouldings to improve the flow of the compound or to produce specified properties.
PIGMENTS: Pigments are used to produce specified colours in rubber. Colours are only usable with a compound which does not contain carbon black.
ANTI-OXIDANTS: Chemicals which are added to the compound to improves resistance of surface attack of the finished product especially by ozone.
PROCESS AIDS: Resins, soaps, low-weight polyethylene
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